The 22-year-old Senegal international had already completed a medical in France on Tuesday evening, with the transfer appearing to be a formality before it unexpectedly fell through shortly after the 23:00 BST deadline passed.
Chelsea had two earlier bids rejected by the Ligue 1 side before Monaco lowered their asking price once Folarin Balogun's proposed move to Everton came back into doubt over medical concerns.
With the Camara deal seemingly agreed, Chelsea had already sanctioned Enzo Fernandez's departure to Manchester City in a joint-British transfer record £125million switch, expecting the Senegalese midfielder to arrive as his replacement.
Instead, the club say they were informed by Monaco in writing that the transfer would not proceed, without any explanation offered for the change of heart.
Sources with knowledge of the situation have told BBC Sport that Monaco's stance is believed to have been influenced by Everton's parallel pursuit of Balogun. Once the American forward pulled out of his own move at the eleventh hour, Monaco no longer needed to complete Camara's sale to balance their finances in the way they had anticipated earlier in the day.
The collapse leaves Chelsea without the midfield reinforcement they had lined up to soften the blow of Fernandez's exit, despite having gone through with that sale on the understanding that Camara's arrival was close to confirmed. It marks a chaotic conclusion to deadline day for a club that had appeared to have its business largely in order, and leaves head coach Xabi Alonso with a thinner central midfield than intended heading into the coming weeks.
Chelsea did explore other midfield options during the closing stages of the window, including Tottenham's Lucas Bergvall, Real Sociedad's Martin Zubimendi and Nottingham Forest's Morgan Gibbs-White, but Camara had emerged as their preferred target before Monaco's late change of heart.
The Blues will now have to wait until January to address the gap left by Fernandez's departure, having banked a fee that comfortably exceeded the £107million they originally paid Benfica for the Argentine three years ago.